Electrical circuits requiring high power handling capability while operating at high frequencies, such as radio frequencies (500 MHz), S-band (3 GHz) and X-band (10 GHz), have in recent years become more prevalent. Because of the increase in high power, high frequency circuits, there has been a corresponding increase in demand for semiconductor devices which are capable of reliably operating at radio and microwave frequencies while still being capable of handling high power loads.
To provide increased output power, semiconductor devices have been developed that include a plurality of “unit cell” transistors that are formed on a common semiconductor structure and that are electrically connected in parallel. Each unit cell transistor may include a gate finger that extends in parallel between elongated source and drain contacts, as is schematically illustrated in FIG. 1.
In particular, FIG. 1 illustrates a metal layout of a conventional semiconductor device 10 that includes a gate pad 12, a source pad 22 and a drain pad 32 on a semiconductor structure 20. FIG. 1 is a plan view of the semiconductor device (i.e., looking down at the device from above) that illustrates various metal contact structures of the semiconductor device 10 that are formed on the underlying semiconductor structure 20. As shown in FIG. 1, in the conventional semiconductor device 10, the gate pad 12 is connected by a gate bus 14 to a plurality of gate fingers 16 that extend in parallel in a first direction (e.g., the y-direction indicated in FIG. 1). The drain pad 32 is connected to a plurality of drain contacts 36 via a drain bus 34. The source pad 22 is connected to a plurality of parallel source contacts 26 via a source bus 24 that is disposed at a different metallization layer (here a higher metallization layer that runs above the gate fingers 16 and the drain contacts 36). Vertically-extending (i.e., extending in a z-direction that is perpendicular to the x-direction and the y-direction) source contact plugs 28 electrically connect each source contact 26 to the source bus 24.
Each gate finger 16 runs along the y-direction between a pair of adjacent source and drain contacts 26, 36. A unit cell transistor of semiconductor device 10 is illustrated at box 40, and includes a gate finger 16 that extends between adjacent source and drain contacts 26, 36. The “gate length” refers to the distance of the gate metallization in the x-direction, while the “gate width” is the distance by which the gate fingers 16 and the source and drain contacts 26, 36 overlap in the y-direction. That is, “width” of a gate finger 16 refers to the dimension of the gate finger 16 that extends in parallel to the adjacent source/drain contacts 26, 36 (the distance along the y-direction). The power handling capability of the semiconductor device 10 may be proportional to its “gate periphery.” The gate periphery of semiconductor device 10 is the sum of the gate widths for each gate finger 16 of the semiconductor device 10.
Semiconductor devices formed of wide band-gap semiconductor materials such as silicon carbide and/or gallium nitride based semiconductor materials may operate at higher current densities and hence are widely used in high power applications. In particular, gallium nitride based transistors that include one or more epitaxial layers of gallium nitride based semiconductor materials such as GaN, AlGaN, InGaN, etc. are now commonly used in high power applications such as transistor amplifiers for wireless communications. These gallium nitride based epitaxial layers are typically grown on silicon carbide or sapphire substrates. There is a need, however, for high power semiconductor devices that exhibit improved performance.